An Approach to Understanding the Electrocatalytic Activity

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An Approach to Understanding the Electrocatalytic Activity Enhancement by Superexchange Interaction Toward OER in Alkaline Media of Ni-Fe LDH Miguel Angel Oliver-Tolentino, Juvencio Vazquez-Samperio, Arturo Manzo-Robledo, Rosa de Guadalupe Gonzalez-Huerta, Jorge Luis Flores-Moreno, Daniel Ramírez-Rosales, and Ariel Guzman-Vargas J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp506946b • Publication Date (Web): 05 Sep 2014 Downloaded from http://pubs.acs.org on September 6, 2014

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The Journal of Physical Chemistry

An Approach to Understanding the Electrocatalytic Activity Enhancement by Superexchange Interaction Toward OER in Alkaline Media of Ni-Fe LDH Miguel A. Oliver-Tolentinoa, Juvencio Vázquez-Samperiob, Arturo Manzo-Robledoc, Rosa de Guadalupe González-Huerta c, Jorge L. Flores-Morenod , Daniel Ramírez-Rosalese and Ariel Guzmán-Vargasb,* Instituto Politécnico Nacional, Centro de Investigación en Ciencia Aplicada y Tecnología Avanzada, Calzada Legaria 694, Col. Irrigación, México D.F. 11500, Mexico. b Instituto Politécnico Nacional, ESIQIE-Departamento de Ingeniería Química, Laboratorio de Investigación en Materiales Porosos, Catálisis Ambiental y Química Fina, UPALM Edif. 7 P.B. Zacatenco, GAM, México, DF 07738, Mexico. c Instituto Politécnico Nacional, ESIQIE-Departamento de Ingeniería Química, Laboratorio de Electroquímica y Corrosión, Edif. Z-5 3er piso, UPALM, Zacatenco, GAM, México, DF 07738, Mexico. d Universidad Autónoma Metropolitana-Azcapotzalco, Área de Química de Materiales, Av.San Pablo180, Col. Reynosa Tamaulipas, 02200,México, DF, Mexico. e Instituto Politécnico Nacional, ESFM-Departamento de Física, UPALM Edif.9 Zacatenco, GAM, México, DF 07738, Mexico. a

Abstract In the present work the hydrotalcite-like materials known as Layered Double Hydroxide (LDH) were synthesized. The Ni-Al and Ni-Fe materials with different Ni/Fe ratio were obtained by co-precipitation method at variable pH. The LDH structure was verified by XRD, FT-IR and Raman spectroscopy. No secondary extra phases were observed for any material. The electronic properties were evaluated by UV-Vis spectroscopy while the magnetic ones were followed by Electron Paramagnetic Resonance (EPR). The results suggested that sample H/Ni-Fe2 (Ni/Fe=2) has a ferrimagnetic behavior as a result of the combined action of NiII-OH-NiII, FeIII-OH-NiII and FeIII-OH-FeIII pairs across the layers and ferromagnetic interactions operating between layers. Furthermore, the material H/Ni-Fe1 (Ni/Fe=1.5) showed a combination of paramagnetic and ferromagnetic interactions which favors a superexchange interaction among metal centers through the OH bridges across the cationic sheets, the superexchange interaction enhances the electrocatalytic activity on the Oxygen Evolution Reaction (OER) in alkaline media. On the other hand, XPS experiments showed that the H/Ni-Fe1 did not exhibit structural changes after electrochemical processes. The activity toward the OER was in the order H/Ni-Fe1>H/Ni-Fe2>H/Ni-Al, as was confirmed using in situ linear sweep voltammetry (LSV) coupled with mass spectroscopy (DEMS). Keywords: Hydrotalcites; Magnetic Properties; Oxygen Production; Electrocatalysis; Characterization; DEMS.

Introduction The efficient production of hydrogen by using renewable energy-resources is a key component in the development of future energy-storage technologies. One method of producing hydrogen is from water electrolysis induced by renewable sources such as sunlight or wind. The efficiency of this process is limited by the oxygen evolution reaction (OER), which occurs simultaneously with hydrogen evolution.1, 2 The hydrogen evolution reaction (HER) is a relatively simple reaction that readily occurs at low overpotential on many metals. Whereas, the oxygen evolution mechanism involves several steps having large reaction barriers, and then leading to large overpotentials to drive the reaction at practical rates. This large overpotential significantly decrease the efficiency, as the extra energy is dissipated as low-quality heat, limiting the possibility of hydrogen-large scale production from water splitting.3, 4 On the other hand, the most active OER catalysts are IrO or RuO2 working in acid or alkaline conditions; however these materials suffer for their scarcity and high cost of precious metals.5 In this context, extensive efforts have been taken to develop highly active, durable, and low-cost alternatives. In particular, the activities of transition-metal-based catalyst for OER were proposed to relate to the 3d electron number; the surface of this metal exhibited eg orbitals which could bond with surface-anion adsorbents. Among the materials that have been studied are perovskites,6 nickel oxides-based materials.7, 8 Specially, the Fe plays a critical, but not yet understood, role in enhancing

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the activity of the Ni-based oxygen evolution reaction (OER) electrocatalysts.9, Hydroxides (LDH) materials have showed promising results.11-15

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10

Recently Layer Double

LDHs belong to the anionic clays family or the hydrotalcite-like compounds. LDHs result from the stacking of positively charged brucite-like octahedral layers. The positive charge results from the replacement of part of the divalent metal (MII) cations by trivalent (MIII) ones. The spaces between layers host solvated anions (An-) that compensate the positive charge of layers. Thus, these materials exhibit specific properties as anion exchangers,16 hydrotalcite-like compounds can be represented by the general formula: [MII1-xMIIIx (OH)2]x+ [An-x/n] • mH2O In the LDH family, the metallic cations can be from the transition group and thus undergo redox reactions in the range of applied potential. This inclusion of transition metals in the layers has been proposed to enhance the charge transport of the material. Such charge transport can be thought as due to a mixed mechanism involving an ‘electron hopping’ along the layers; which is ascribable to an inner redox reaction between oxidized and reduced forms of the MII/MIII couple, and a migration of anions inside the interlayers to compensate the positive extra-charge.17,18 In particular, the Ni-based LDH has been studied as electrochemical sensor,19 electrochemical pseudo capacitor,20 and electrocatalyst.21, 22 Nevertheless, few works, involving NiFe LDH, devoted to water electrolysis has been published. Recently Gong et al. reported the performance of LDH Ni-Fe (Ni/Fe=5), the main results pointed out high electrocatalytic activity and stability for oxygen evolution reaction in alkaline media than commercial precious metal-based catalysts.11 Lu et al. showed that 3D architecture films of vertically aligned NiFe-LDH nanoplates exhibited excellent performance in the OER: small onset overpotential, low Tafel slope, large anodic current density and prominent electrochemical durability.12 Tang et al. synthesized carbon quantum dot/Ni-Fe LDH nanoplates, this complex system exhibited high catalytic activity toward water electrolysis and good stability for oxygen evolution.13 Long et al. reported a low overpotential (as low as 0.195 V) of catalytic activity towards OER using hybrid catalyst of NiFe-LDH and Graphene.14 However, until now it has not been reported some explanation that allow us to understand the activity towards the OER from the LDH properties. In the study herein, the synthesis of Ni-Al and Ni-Fe LDH with different Ni/Fe ratio is reported. The materials were evaluated as electrocatalysts in Oxygen Evolution Reaction and their activity was explained from the NiFe material magnetic behavior. The OER takes place with the highest faradic performances as also demonstrated by the in-situ differential electrochemical mass spectroscopy (DEMS) technique.

1.

Experimental Section

The Ni-Al and Ni-Fe LDH were prepared by coprecipitation as described elsewhere. As an example, H/NiFe2 was prepared by dissolving 0.117 moles of NaOH (Aldrich) and 0.034 moles of Na2CO3 (Aldrich) in 100 mL of deionized water; the pH of this solution was 13.4. A second solution was prepared by dissolving 0.034 moles of Ni(NO3)2·6H2O (Aldrich) and 0.017 moles of Fe(NO3)3·9H2O (Aldrich) in 100 mL of deionized water. While maintaining the first solution under vigorous stirring the second solution was slowly added by means of a peristaltic pump. After complete addition the resultant slurry was stirred for 2 h at room temperature; the pH of the suspension was 9.5. Finally, the suspension was stirred for 2 days at 50 °C, and then the solid obtained was separated by centrifugation, rinsed thoroughly with warm distilled water, and dried overnight at 80 °C. The solids obtained were labeled H/Ni-Fe1 and H/Ni-Fe2 with Ni/Fe ratio 1.5 and 2 respectively, and H/Ni-Al, Ni/Al=4. The modified carbon paste electrode were prepared mixing graphite powder (Alfa Aesar, 99.9995%, USA), silicon oil (Aldrich) and the corresponding LDH at 20 wt.%. The mixture was mechanically homogenized and


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inserted in 2 mm diameter cylinder (0.0314 cm2). The surface contact on the electrode was made with a platinum wire. The XRD structural characterization of LDHs was performed in a Philips X’PERTPRO instrument using CuKα1 radiation (λ= 1.542 Å, 45 kV, and 40 mA); whereas the spectroscopic analyses were recorded in a FTIR spectrophotometer Perkin Elmer RX-1, with attenuated total reflectance accessory (ATR) and a Raman spectrophotometer Labram HR800 with a laser operating at 784.29 nm. XPS studies were performed by using a Thermo Scientific KAlpha X-ray photoelectron spectrometer with a monochromatized AlKα X-ray source (1,487 eV). Narrow scans were collected at 60 eV analyzer pass energy and a 400 µm spot size. The electronic properties were performed with a UV-Vis-NIR spectrophotometer Perkin Elmer Lambda 950. Electron Paramagnetic Resonance (EPR) measurements were carried out at room temperature and 77 K using a JEOL JES-RES3X continuous wave EPR spectrometer. Typical EPR spectral parameters were as follow: X-band frequency = 9.1642 GHz, modulation amplitude = 3.2 G, modulation frequency = 100 kHz. Electrochemical analyses were carried out at room temperature in a potentiostate–galvanostate VERSASTAT3-400 (Princeton Applied Research). A three-electrode standard electrochemical cell was used for the cyclic voltammetry (CV) measurements at 5 mV s-1 with a carbon rod and a Saturated Calomel Electrode (SCE), respectively. For these experiments, the working electrode was made from the synthesized materials, immersed in a carbon paste electrode (CPE) matrix. For in-situ experiments during polarization in the OER region, Differential Electrochemical Mass Spectrometry (DEMS) coupled with a home-made electrochemical cell made of Teka-Peek was used. Briefly, the electrochemical cell was connected to the chamber containing the quadrupole mass spectrometer (MS, Prisma QMS300, Pfeiffer) through a precision valve, which allows the isolation of the ion source from the electrochemical cell forming a small pre-chamber. The DEMS system allows using three electrochemical cells connected to the vacuum system. A duo pumps evacuates the latter, whereas the vacuum in the chamber containing the MS is obtained from a turbo molecular pump (the working pressure was ca. 7×10−6 mbar for all set of experiments described therein). The amount of species reaching the MS, throughout a porous membrane (60 µm thick, 0.2 µm pore diameter, and 50% porosity) can be controlled with the dosing valve located between the electrochemical cell and the prechamber. Mass spectrometric profiles (ionic current (Ii) versus potential (E)) and faradic current (IF) versus potential (E) for selected mass to charge ratios (m/z) were recorded simultaneously. The cell DEMS also allows for the renewal of electrolyte via a peristaltic pump (e.g. an electrolyte flow rate of 0.5mL/min). For all set of experiments, linear voltammetry was reported with compensated cell resistance (iR). The Reversible Hydrogen Electrode (RHE) scale, was calculated based on the formula: VRHE = VSCE + VSCE (vs NHE) + (0.059* pH)

2.

Results and discussion

Materials Characterization Lamellar structure of solids was confirmed by X-ray diffraction, infrared spectroscopy and Raman spectroscopy. The powder X-ray diffractograms of the synthesized samples are shown in Figure 1. All samples exhibited the typical signature of hexagonal lattices with R3m rhombohedral symmetry (JCPDS card 22-0700), 21 which are similar to diffraction patterns of α-Ni(OH)2 phase as reported in ref. 11. The calculated cell parameters from X-ray data and the molar ratio between MII/MIII are listed in Table 1. The c parameter corresponds to three times the interlayer distance d which is obtained from the 003 diffraction peak. The cell parameter, a, stands for the cation–cation average distance inside the layers and it can be determined from the angular position of the 110 reflection. The observed values for the c parameter are typical of LDH containing carbonate as the interlamellar anion. It has



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been reported that the interlayer distance depends on the size and the orientation of the interlayer anions as well as on the thickness and charge density of the layers. The layer thickness strongly depends on the size of the cations.23 It turns out that, as the charge density is increased, the c parameter decreases because the attractive coulomb forces have been increased between the positive (layers) and negative (interlayer region) charged regions.23 Figure S1A (supplementary information) shows the FT-IR spectra for LDH materials. The intense and broad lines are observed between 3416 and 3388 cm-1 and ascribed to the O-H stretching vibration mode of water molecules which are intercalated within the interlaminar space. Besides interlaminar water molecules, hydrogen-bonded hydroxyl groups along the cationic sheets might also be responsible for broadening the band. The medium-intensity band around 1630 cm-1 is related to the bending mode of those hydrogens bonded to water molecules. In fact, this band intensity is proportional to the amount of intercalated water in the studied sample. The stretching mode of CO32- anions is responsible for the sharp and strong band around 1360 cm-1. The stretching and bending vibrational modes were attributed to hydroxometallic octahedral complexes which constitute the [MII1-xMIIIx (OH)2]x+ cationic brucite-like sheets are responsible for absorption at lower wavenumbers (H/Ni-Fe2>H/Ni-Al. On the other hand, the current-potential characteristic obtained using cyclic voltammetry (CV) for H/Ni-Al displays well-defined anodic and cathodic peaks associated with redox process for the couple NiIII / NiII with a peak-to-peak potential ∆Ep=200 mV, see inset in Figure 3. This electrochemical behavior is well-known and is due to insertion/desertion of OH- ions from the LDH-interlayer space during nickel-sites oxidation/reduction by electron hopping mechanism along the brucite structure inducing electro-neutrality. Then, the redox process can be represented by reaction (R1):18 H/NiII-Al + OH-sol

H/NiIII (OH-)ie-Al + e-

(R1)

The CV for LDH materials are exhibited in Figure S2, from these profiles an amplification ranged between 1.2 and 1.4 V/RHE, showed the faradic peak at 1.3 V/RHE associated to reduction of NiIII to NiII during cathodic scan (see inset), suggesting that in the H/Ni-Fe solids also nickel oxidation takes place during anodic scan. It is worth to mention that non-faradic processes were observed at CPE free of LDH (Figure not shown). In order to confirm the generation of molecular oxygen during anodic polarization in the LDH-based materials, the in-situ DEMS technique was employed. For such an approach an alternative electrochemical



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cell connected to the MS was used. The scan rate was fixed at 1 mV s-1. Figure 4 A-D shows the currentpotential characteristics corresponding to the OER onto the LDH, in argon-purged 1 M KOH solution. Notices that, for the sample H/Ni-Fe1 the magnitude of the faradic current (Figure 4A) is higher when it is compared with sample H/Ni-Fe2 (Figure 4D). This faradic current represents, in fact, the production of oxygen at the interface of the electrode, as it can be observed from the mass-to-charge ratio (m/z=32) obtained by DEMS in Figure 4B and 4E for H/Ni-Fe1 and H/Ni-Fe2, respectively. However, such mass signal is higher for H/Ni-Fe1 as expected for the magnitude of faradic current from Figure 4A. In addition, for both samples analyzed in Figure 4, no evidence of secondary redox reaction (i.e. corrosion) was observed as the mass signal corresponding to carbon dioxide (m/z=44) was not perturbed, see Figure 4C-F. In this context, for the case of sample H/Ni-Al and CPE free of LDH, the magnitude of the faradic current is less intense and then the production of oxygen decreases (Figure S3). The DEMS analysis then puts nicely in clear that the electrocatalytic activity toward OER is in the order H/Ni-Fe1>> H/N-Fe2> H/Ni-Al>CPE. Tafel plots obtained from polarization curves were fashioned (Fig. 5A). The resulting Tafel slopes were ca. 34, 36 and 37 mV dec-1 for H/Ni-Al, H/Ni-Fe2 and H/Ni-Fe1, respectively. These values are smaller than those reported for the system Ir/C which exhibited a Tafel slope of ca. 40 mV dec-1;11 indicating that, within experimental error, the OER mechanism is similar for all set of these materials. Such reaction mechanism might be related with i) a surface oxidation by one electron electrochemical step (R2); ii) adsorption step (R3); and iii) a one electron-electrochemical rate-determining step for oxygen production (R4),7 general process suggested is described by the next reactions. H/NiII-MIII + OH-sol H/NiIII (OH-)ie-MIII + OH-sol

H/NiIII (OH-)ie-MIII + eH/NiIII (OH-)ie-MIII(OH)ad + e-

H/NiIII (OH-)ie-MIII(OH)ad + OH-sol

H/NiIII (OH-)ie-MIII + ½ O2 + H2O + e-

(R2) (R3) (R4)

Where, sol: solution, ie: interlayer space, ad: adsorbed and M: is FeIII or AlIII. On the other hand, the specific electro-catalytic performance of the system can be given in terms of the turnover number (TON), defined here as the number of oxygen molecules generated at 1.62 V per second and per catalytic site. The TON number was calculated (see supplementary information) assuming that all available nickel-sites are involved in the electrochemical reaction.8 The TON values obtained were H/Ni-Al ~0.05, H/Ni-Fe2 ~9.93 and H/Ni-Fe3 ~38.08 s-1, whereas the current at a potential of 1.65V/RHE were 46, 120 and 922 µA for H/Ni-Al, H/Ni-Fe2 and H/Ni-Fe1 respectively. These values put in evidence that, as discussed above, H/Ni-Fe1 exhibited the highest electro-catalytic activity toward OER; attributed to increase the amount of Fe which induced partial-charge transfer mechanism, which activates Ni centers.10 This could be explained due to electron delocalization on the layer surface, which permits the improved of electron hopping through brucite layer. Such electron delocalization is due to the arrangement of the Fe atoms in the lattice, which spins form a specific magnetic structure that favors the superexchange interaction. A greater superexchange interaction implies a greater electron delocalization. The effect of anionic interference in the O2 production for H/Ni-Fe1 material, using Cl- and NO3- in a ratio 2:1 with respect to OH-, is observed in Figure 5B. The results showed that the Tafel slope decrease in presence of Cl- and NO3- (see Inset). Similar behavior was observed for H/Ni-Fe2. This fact can be explained in terms of i) insertion in the interlayer space or ii) adsorption on the layer of Cl- or NO3- during the anodic polarization. However, due to the strong interaction of OH- with the brucite layers and the prioritization of anionic exchange, only the insertion of OH- could be favored by the application of an anodic potential. This was verified by cyclic voltammetry profiles for H/Ni-Al at the same solution that in Figure 5 (Figure not shown). From this, the i-E characteristics did not exhibited significant differences in shape of the signal peak potential


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related with oxidation and reduction of nickel centers. However, some changes in the reaction-mechanism were observed, verifying that OER occurs on the brucite layers, according to reactions R2-R4. In addition, the current obtained for H/Ni-Fe1 at 1.63V/NHE was more intense in solution containing Cl- + OH- (curve c, Fig 6.). Whereas, lower current intensity was attained using solutions based on NO3- + OH- ions (curve b) and with OH- free of NO3- and Cl- (curve a). This behavior might be associated to the adsorption of Cl- on the layer-surface producing the oxidation of Cl- to Cl2. Conversely, at solution based on NO3- ions more studies are necessary in order to major understand the implied phenomena causing lower faradic efficiency. The TON value calculated were 25 s-1 in Cl- and 17 s-1 in NO3- indicating that current due to OER decreases. Furthermore, the stability of H/Ni-Fe1 structure after the induced electrochemical process was evaluated by XPS (Figure 6). The spectra of the raw sample (i.e. before induced-electrochemical reaction) (curve a) show the typical binding energy corresponding to Ni 2p3/2 and 2p1/2 located at c.a. 855.5 and 873.2 eV respectively (Figure 6A). Whereas the binding energy for Fe 2p3/2 and 2p1/2 are positioned at 712 and 725.3 eV (Figure 6B); confirming the oxidation state (2+) for Ni and (3+) for Fe. In addition, using high resolution XPS, nickel species were identified and qualitatively analyzed using the decomposition model propose by Grosvenor et al.,32 including nickel hydroxide and oxyhydroxide spectra (Figure S4). Ni2p doublet spectrum of each sample was reconstructed using a Gaussian–Lorentzian mix function and Shirley background subtraction. To decompose the experimental spectrum it was necessary to use seven double contributions: Ni(OH)2 located at 454.9 and 872.5 ± 0.2 eV, for Ni 2p3/2 and Ni 2p1/2 peaks, respectively. While Ni2+multiplets and satellites contributions were located taking into account the results reported in ref. 32. The high resolution XPS-spectra after fifty cycles using cyclic voltammetry did not show important structural modifications. 3. Conclusion LDH materials with different Ni/Fe ratio using co-precipitation method at variable pH were synthetized. The iron content in the structure of these materials plays an important role on their magnetic properties. It may be postulated that the superexchange interaction is one of the main causes that promotes the enhancement of the H/Ni-Fe1 catalytic activity toward OER, the catalytic activity associated to TON value of this sample was higher than H/Ni-Fe2. Production of molecular oxygen (m/z=32) and the absence of secondary interfacial-electrochemical reactions during anodic polarization (i.e. corrosion) were confirmed using LSV coupled with mass spectroscopy (DEMS). Whereas, Tafel slopes were around 40 mV dec-1, indicating that every materials rules out by the same mechanism reaction, where one electron electrochemical is the rate determining step for O2 production. On the other hand, experiments carried out at different anion-based solutions did not showed important catalytic effect on OER. The good stability of LDH materials after electrochemical process was observed by XPS spectra. This study affords identify the importance of magnetic behavior in the electrochemical process. 4.

Author Information Corresponding author

Instituto Politécnico Nacional, ESIQIE-Departamento de Ingeniería Química, Laboratorio de Investigación en Materiales Porosos, Catálisis Ambiental y Química Fina, UPALM Edif. 7 P.B. Zacatenco, GAM, México, DF 07738, Mexico. Tel: +52 5557296000, Email: [email protected] Notes The authors declare no competing financial interest.

5.

Acknowledgments

This work was partially supported by Projects: SIP-IPN 20140793, CONACYT 101319, SECITI DF (ICVIDF/193/2012). AM-R thanks IPN-CONACYT 160333-DEMS.



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6.

Supporting Information Available

Figure S1. A) FTIR and B) Raman spectra of a) H/Ni-Al, b) H/Ni-Fe1 and c) H/Ni-Fe2. Figure S2. Cyclic Voltammetry of a) H/Ni-Al, b) H/Ni-Fe1 and c) H/Ni-Fe2 in 1M KOH; inset Cathodic sweep. Figure S3. DEMS profiles of A) H/Ni-Al and B) CPE, in 1M KOH. Figure S4. XPS spectra of Ni2p deconvolution, and Calculation of Turnover Number (TON). This information is available free of charge via the Internet at http://pubs.acs.org 7.

References

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11.- Gong, M.; Li, Y.; Wang, H.; Liang, Y.; Wu, J. Z.; Zhou, J.; Wang, J.; Regier, T.; Wei, F.; Dai, H.; An Advanced Ni-Fe Layered Double Hydroxide Electrocatalyst for Water Oxidation. J. Am. Chem. Soc. 2013, 135, 8452-8455. 12.- Lu, Z.; Xu, W.; Zhu, W.; Yang, Q.; Lei, X.; Liu J.; Li, Y.; Sun, X.; Duan, X.; Three Dimensional NiFe Layered Double Hydroxide Film for High-efficiency Oxygen Evolution Reaction. Chem Comm. 2014, 50, 6479-6482. 13.- Tang, D.; Liu, J.; Wu, X.; Liu, R.; Han, X.; Han, Y.; Huang, H.; Liu, Y.; Kang, Z.; Carbon Quantum Dot/NiFe Layered Double-Hydroxide Composite as a Highly Eﬃcient Electrocatalyst for Water Oxidation. ACS Appl. Mater. Interfaces 2014, 6, 7918-7925. 14.- Long, X.; Li, J.; Xiao, S.; Yan, K.; Wang, Z.; Chen, H.; Yang, S.; A Strongly Coupled Graphene and FeNi Double Hydroxide Hybrid as an Excellent Electrocatalyst for the Oxygen Evolution Reaction. Angew. Chem. Int. Ed. 2014, 53, 1–6. 15.- Zhang, Y.; Cui, B.;Zhao, C.; Lin, H.; Li, J.; Co–Ni Layered Double Hydroxides for Water Oxidation in Neutral Electrolyte. Phys. Chem. Chem. Phys. 2013, 15, 7363-7369. 16.- Miyata, S.; Anion-Exchange Properties of Hydrotalcite-Like Compounds. Clays Clay Miner. 1983, 31, 305–311. 17.- Scavetta, E.; Berrettoni, M.; Giorgetti, M.; Tonelli, D.; Electrochemical Characterisation of Ni/Al-X Hydrotalcites and their Electrocatalytic Behavior. Electrochimica Acta 2002, 47, 2451-2461. 18.- Aguilar-Vargas, V.; Sánchez-Valente, J.; González, I.; Electrochemical Characterization of Carbon Paste Electrodes Modified with MgZnGa and ZnGaAl Hydrotalcite-Like Compounds. J. Solid State Electrochem. 2013, 17, 3145-3152. 19.- Ai, H.; Huang, X.; Zhu, Z.; Liu, J.; Chi, Q.; Li, Y.; Li, Z.; Ji, X.; A Novel Glucose Sensor Based on Mono Dispersed Ni/Al Layered Double Hydroxide and Chitosan. Biosens. Bioelectron. 2008, 24, 1048-1052. 20.- Wang, B.; Liu, Q.; Qian, Z.; Zhang, X.; Wang, J.; Li, Z.; Yan, H.; Gao, Z.; Zhao, F.; Liu, L.; Two Steps in situ Structure Fabrication of Ni-Al Layered Double Hydroxide on Ni Foam and its Electrochemical Performance for Supercapacitors. J. Power Sources 2014, 246, 747-753. 21.- Oliver-Tolentino, M.A.; Guzmán-Vargas, A.; Manzo-Robledo, A.; Martínez-Ortiz, M.J.; Flores-Moreno, J.L.; Modified Electrode with Hydrotalcite-Like Materials and their Response During Electrochemical Oxidation of Blue 69. Catal. Today 2011, 166, 194-200. 22.- Mignani, A.; Ballarin, B.; Giorgetti, M.; Scavetta, E.; Tonelli, D.; Boanini, E.; Prevot V.; Mousty, C.; Iadecola A.; Heterostructure of Au Nanoparticles-NiAl Layered Double Hydroxide: Electrosynthesis, Characterization, and Electrocatalytic Properties. J. Phys. Chem. C 2013, 117, 16221-16230. 23.- Iglesias, A.H.; Ferreira, O.P.; Gouveia, D.X.; Souza Filho, A.G.; de Paiva, J.A.C.; Mendes Filho, J.; Alves, O.L.; Structural and Thermal Properties of Co–Cu–Fe Hydrotalcite-Like Compounds. J. Solid State Chem. 2005, 178, 142-152.



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Figure Caption and Table

Figure 1. X-Ray Diffraction patterns of a) H/Ni-Al, b) H/Ni-Fe1 and c) H/Ni-Fe2

Figure 2. A) UV-Vis-NIR spectra of a) H/Ni-Al, b) H/Ni-Fe1 and c) H/Ni-Fe2; B) EPR spectra of a) H/NiFe1 and b) H/Ni-Fe2, black line at room temperature and blue line at 77K.

Figure 3. Linear Sweep Voltammetry of a) H/Ni-Al, b) H/Ni-Fe1 and c) H/Ni-Fe2, inset: cyclic voltammetry of H/Ni-Al in 1M KOH.

Figure 4.Current-potential characteristics for A) H/Ni-Fe1 and D) H/Ni-Fe2, in 1M KOH and scan rate of 1mV s-1. B-C and E-F are the mass signal profiles obtained using DEMS during anodic polarization.

Figure 5. A) Tafel Plots of a) H/Ni-Al, b) H/Ni-Fe1 and c) H/Ni-Fe2 obtained from i-E characteristic of Fig. 3; B) Linear Sweep Voltammetry of H/Ni-Fe1 in a) 1M KOH, b) 1M KOH + 0.5M KNO3 and c) 1M KOH+ 0.5M KCl, inset: Tafel Plots

Figure 6. XPS spectra A) Ni2p and B) Fe2p for H/Ni-Fe1 a) before to electrochemical process and after 50 cycles in different solution b) 1M KOH, c) 1M KOH + 0.5M KNO3 and d) 1M KOH+ 0.5M KCl.



Figures

Relative Intensity (a.u.)

(003)

(110) (113)

(018)

(015)

(009)

(006)

c

b a 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 2θ (degrees)

Figure 1

1.0

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ν5

A

ν4

a

B

0.8 ν2

0.6 0.4

sf1

500 G

ν1

c ν3

b

b sf2

0.2

a

500 G 0.0 200

400

600

800

1000

1200

λ (nm)

Figure 2


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1800

b

100

1600

80

1400 i (µ A)

60

i (µ A)

1200

40 20

1000

0

800

-20 0.8

600

1.0

1.2

1.4

1.6

1.8

E (V vs RHE)

400

c

200

a

0

0.8

1.0

1.2

1.4

1.6

1.8

E (V vs RHE)

Figure 3

m/z=44 (CO2)

5 pA

C m/z=32 (O2)

mass signal

mass signal

m/z=44 (CO2)

0.02 nA

F m/z=32 (O2) 0.2 nA

1 nA

B

500 µA

A 0.6

0.8

1.0

1.2

1.4

1.6

1.8

E Current Intensity

Current Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


200 µA

D 0.6

0.8

1.0

1.2

E / V (RHE)

E (V vs RHE)

Figure 4


1.4

1.6

1.8


1800 1600

1000 -1

1500

i (µA)

1200

-1

100

ec 36 mV d

c

-1

1400

Ik(µ A)

37 mV dec

Ik (µA)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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30 mV dec

1200

b

-1

33 mV dec

c 1000

-1

b

900

800

37 mV dec

a

1.59

1.60

1.61

600

1.62

1.63

1.64

1.65

a

E(V vs RHE)

-1

34 mV dec

300

A

B 0

1.60

1.62

1.64

1.66

1.68

1.70

0.9

1.0

1.1

1.2

1.3

1.4

E (V vs RHE)

E (V vs RHE)

Figure 5


1.5

1.6

1.7

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A

B

Ni2p3/2

2+

Ni

Ni2p1/2

2+

(Sat)

Ni

Fe2p3/2

Fe2p1/2

a (Sat)

b c d

886

881

876

871

866

861

856

851

735

730

725

720

715

Binding energy (eV)

Binding energy (eV)

Figure 6


710


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Table 1. Divalent and trivalent cation ratio and cell parameters. Sample MII/MIII * d003(Å) 4.0 7.513 H/Ni-Al 1.5 7.481 H/Ni-Fe1 2.0 7.596 H/Ni-Fe2 *Composition obtained by XPS analysis

d110(Å) 1.511 1.535 1.540


a(Å) 3.023 3.070 3.080

c(Å) 22.538 22.443 22.789

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TOC


An Approach to Understanding the Electrocatalytic Activity

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